Method and apparatus for testing of sheet material

ABSTRACT

An apparatus for testing of electrical, mechanical, physical and/or chemical properties of material in sheet form includes a cassette for holding of sheets and sensors rigidly mounted relative to the cassette. The sensors may be mounted adjacent a test location exterior to the cassette. The cassette and the sensors may be so configured and positioned that a suitable end effector may move sheets of material between storage locations in the cassette and test locations adjacent the sensors. A method for testing sheet material includes the steps of placing the sheet material in a cassette, and conducting tests employing one or more sensors rigidly mounted with respect to the cassette. The method may include employing an end effector to remove the sheet from the cassette, to position the sheet stepwise in several positions relative to the sensors, and to replace the sheet in the cassette upon completion of testing. Materials to be tested include flat panels for computer screens and other applications and semiconductor wafers.

RELATED APPLICATIONS

This application is a continuation of U.S. Patent Application No.09/316,677, filed May 21, 1999, now U.S. Patent No. 6,202,482, which isa continuation-in-part of U.S. Patent Application No. 09/274,487, filedMarch 23, 1999, now U.S. Patent No. 6,205,852, which claims priorityfrom U.S. Provisional Patent Application No. 60/079,058, filed March 23,1998, which application is incorporated by reference herein.

FIELD OF THE INVENTION

This application relates to the field of testing of sheet materials, andin particular testing of thin glass sheets, such as glass sheets for usein computer flat panel displays, or for semiconductor wafers.

BACKGROUND OF THE INVENTION

Glass sheets or panels are conventionally maintained, after fabricationof the glass sheets, and before assembly into products, such as flatpanel displays, in cassettes. Similarly, semiconductor wafers are placedin cassettes. Cassettes are essentially boxes sized to accommodatesheets or cassettes of a selected size. In one existing design ofcassettes, there are provided projecting inward from the sides of thecassette panel supports. A defined distance separates the panelsupports. The defined distance is selected to permit an end effector ofa robot to pass between the panel supports, so as to remove or insertthe panel in the cassette.

After fabrication, and after various steps during processing, panels,semiconductor wafers, and other materials in sheet form, are tested fora variety of physical, electrical, mechanical and chemical properties.Typically, upon fabrication, the panels or wafers are placed in thecassette by a robot with an end effector that engages the panel or waferin such a manner as to minimize damage. When it is desired to test thepanel or wafer, an end effector of a robot is inserted into thecassette, engages the panel or wafer, and transports the panel or waferfrom the cassette to a testing device. The robot then places the panelor wafer on suitable supports on the testing device. A handler in thetesting device moves the panel or wafer relative to test heads thatcarry out various tests on the panel or wafer. Upon completion oftesting, the robot again engages the panel or wafer and removes thepanel or wafer from the test equipment and returns it to the cassette.

This presents several difficulties. Testing time includes time to removethe panel from the cassette and transport it to the test device, and thetime required to remove the panel from the test device and return it tothe cassette. Each time the panel or wafer is engaged or disengaged byhandling equipment, such as the robot end effector or the handler of thetest device, there is a risk of damage. The time required to positionthe sample or sheet before measurement or testing and replace the sampleafter measurement or testing is increased by the need to have severaldevices successively engage and release the wafer or panel.

OBJECTS AND ADVANTAGES OF THE INVENTION

It is an object of the invention to provide a method and apparatus fortesting of physical, chemical, electrical and mechanical properties ofmaterial in sheet form, such as panels and wafers, that reduces theprocess time associated with testing the material.

It is a further object of the invention to provide a method andapparatus for testing of material in sheet form that reduces the risk ofdamage associated with testing of the material.

It is an advantage of the invention that the foregoing objects areachieved.

Additional objects and advantages of the invention will become evidentfrom a review of the detailed description which follows.

SUMMARY OF THE INVENTION

An apparatus for testing of material in sheet form includes a cassetteadapted to store one or more sheets of material and one or more sensorsrigidly mounted with respect to the cassette. The sensors may be mountedadjacent a test location exterior to the cassette. The cassette and thesensors may be so configured and positioned that a suitable end effectormay move sheets of material between storage locations in the cassetteand test locations adjacent the sensors.

A method for testing sheet material includes the steps of placing thesheet material in a cassette, and conducting tests employing one or moresensors rigidly mounted with respect to the cassette. The method mayinclude employing an end effector to remove the sheet from the cassette,to position the sheet stepwise in several positions relative to thesensors, and to replace the sheet in the cassette upon completion oftesting.

An apparatus for testing of material in sheet form includes sensors thatcan be positioned adjacent to a surface of material in sheet formlocated in a cassette and supports positioned to reduce sag of thematerial.

A method for testing sheet material includes the steps of placing thesheet material in a cassette, and testing the sheet material while inthe cassette.

A cassette according to the invention includes shelves having definedtherein test heads for testing properties of material in sheet form.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a somewhat schematic drawing of an apparatus of the inventionpositioned relative to an exemplary cassette with an exemplary panel inthe cassette.

FIG. 2 is a partial sectional view along line 2—2 of an apparatus ofFIG. 1.

FIG. 3 is a somewhat schematic representation of an alternativeembodiment of an apparatus of the invention.

FIG. 4 is a partial plan view of the apparatus of FIG. 3.

FIG. 5 is a somewhat schematic representation of an alternativeembodiment of an apparatus of the invention.

FIG. 6 is a top plan view of a substrate of the invention of FIG. 5.

FIG. 7 is a sectional view of the substrate of FIG. 6.

FIG. 8 is a top view of an alternative embodiment of the substrate ofFIG. 6.

FIG. 9 is a sectional view taken along line 9—9 of FIG. 8.

FIG. 10 is a bottom plan view of the substrate of FIG. 8.

FIG. 11 is a plan view of an alternative design of the upper surface ofa substrate of the invention.

FIG. 12 is a somewhat schematic representation of an alternativeembodiment of the invention.

FIG. 13 is a top plan view of an alternative embodiment of a cassetteshelf arrangement according to the invention.

FIG. 14 is a partial front cross-sectional view of the embodiment shownin FIG. 13.

DETAILED DESCRIPTION OF THE EMBODIMENTS.

Referring to FIG. 1, there is shown an apparatus 10 according to theinvention. Apparatus 10 is an end effector of a robot arm. More broadly,apparatus 10 is a movable unit. Apparatus 10 is generally a flattenedend effector, relatively thin in height compared to length and width.Apparatus 10 is relatively thin in height in this embodiment to permitapparatus 10 to fit beneath a panel or wafer to be tested and over alower panel or wafer supported in the cassette with adequate clearance.The height of apparatus 10 is dictated principally by the need to clearthe next lower panel or wafer in the cassette.

Apparatus 10 has three projecting fingers 15. Each finger 15 has asensor or test head 20. The number of fingers 15 may be varied asdesired depending on the test pattern to be achieved. Each sensor 20 issuitable for positioning adjacent to a panel or wafer positioned in acassette. Sensor 20 may be any one of numerous types of known compacttest heads for testing properties of sheet materials. A wide variety ofphysical, chemical, mechanical and electrical properties of materialsmay be tested by use of suitable test heads. For example, the test headsmay provide for eddy current sheet resistance testing. Photoreflectancetesting may be provided. If photoreflectance testing is provided, thenoptical fibers are provided to the test heads. Suitable circuitry andwiring are provided in the body of apparatus 10 to permit communicationbetween sensor 20 and controllers and data storage and read out devices.Communication may include control signals sent to sensor 20, and datasignals received from sensor 20. Three sensors 20 are shown merely asexamples. The number of sensors may be selected as desired, depending onthe number of locations to be tested at any one time.

As an alternative, pairs of coplanar sensors and detectors for eddycurrent detection may be provided in place of each individual sensor 20.

Disposed on opposite sides of sensor 20 are devices 25 for engaging asheet of material. Devices 25 may be small vacuum heads, or vacuum holddowns, as are well known in the art. Devices 25 serve to preventrelative movement between the panel and sensor 20 while testing iscarried out. Air lines (not shown) are provided between vacuum heads 25and electrically controlled valves (not shown). Device 25 may alsosupport the panel. As there may be sag in the panel, which is otherwisesupported only by the panel supports, devices 25 may serve to reducesag. This may also serve to reduce stresses in the panel that resultfrom sag.

FIG. 1 also shows a cassette 30 having panel or sheet 35 therein.Cassette 30 is generally a box, having a planar horizontal bottom,planar vertical parallel sides, a planar horizontal top, and a planarback wall. A front opening is provided opposite the back wall. The termcassette as used herein includes any container for retaining andprotecting multiple sheets of material, such as panels or wafers. Panelsupports 40 project inward from the two side walls. FIG. 1 shows onlyone set of panel supports 40. For convenience of viewing panel supports40 are shown through panel 35. Cassette 30 has numerous such supports.Numerous panels are supported in a horizontal position in cassette 30.

In a method according to the invention, apparatus 10 is mounted on arobot arm. By robot arm, any device capable of supporting and preciselymoving and locating apparatus 10 is meant. Apparatus 10 is positionedrelative to a sheet 35 in cassette 30. Initially, apparatus 10 ispositioned so that testing may be conducted on sheet 35 at the set oflocations closest to the opening of the cassette 30, i.e., at locations50. Preferably, apparatus 10 is moved into position beneath sheet 35,and then moved upward to contact sheet 35, and then moved upwardslightly further a distance sufficient to significantly reduce sag insheet 35. This last step may involve movement upward of about 5 mils,although the precise distance may vary depending on the distance betweensheets and the susceptibility of the particular material and thicknessto sag. Apparatus 10 remains with sensors 20 at locations 50, anddevices 25 holding sheet 35, to prevent relative movement of sensors 20to sheet 35. After the test is completed, devices 25 release sheet 35.Apparatus 10 is moved to the next selected set of locations. The processis repeated. Devices 25 engage sheet 35. Sensors 20 carry out testing onsheet 35 at locations 55. Devices 25 disengage sheet 35, and the processis repeated until all desired locations are tested. Of course, the orderof testing of various locations on the sheet may be varied. Apparatus 10must be sufficiently long to permit sensors 20 to contact the desiredtest location closest to the back wall.

Fingers 15 are shown to provide a certain amount of independent verticalpositioning for the three sensors 20. Apparatus 10 may be constructedwithout fingers, and all sensors 20 supported on a single surface.

The number and relative location of sensors 20 may be selected asdesired.

In summary, in the embodiment of FIGS. 1 and 2, there is provided arobot end effector 10 having one or more sensors 20 or test devicesthereon. The end effector 10 is dimensioned to fit between sheets ofmaterial 35 in a cassette 30 for holding numerous flat sheets ofmaterial. The end effector's length is sufficient to provide testingover all or a substantial portion of a sheet fully inserted in thecassette without moving the sheet. Multiple sensors may be provided onthe end effector, and devices may be provided to engage the sheet toprevent relative movement of sensors and sheet during testing. The endeffector is dimensioned to fit between panel supports projecting fromopposite sidewalls of the cassette.

In a method of the invention in accordance with the foregoing, an endeffector with one or more sensors mounted thereon is inserted in acassette holding sheets of material, and is moved relative to one of thesheets so that the sensor can test the material at selected locations onmany points on the surface of the sheet. This process may be repeatedfor all sheets in the cassette.

Such an effector may be provided, for example, on a Gencobot 7 or 8 GPRrobot, available from Genmark.

Referring now to FIGS. 3 and 4, there is shown an alternative endeffector 110 according to the invention. End effector 110 is shown inFIG. 3 in proximity to cassette 130 which has an exemplary flat panel135 to be tested located therein and supported on supports 140. Endeffector 110 has an array of sensors 150. Sensors 150 may be anysuitable sensor or test head, as discussed above in connection with FIG.1. Sensors 150 are arranged linearly on each finger 115. Vacuum holddowns 120 are provided adjacent to sensors 150. Sensors are multiplexedto external control electronics and electronics for detecting andstoring readings from sensors 150.

In operation, end effector 110 is brought into contact with the sheetmaterial by movement of a robot arm (not shown). End effector 110 is sopositioned relative to the sheet material that sensors 150 arepositioned adjacent to a substantial portion of the surface of the sheetmaterial. As a result, in a single positioning, sensors 150 may conductappropriate tests on the sheet material. Preferably, end effector 110 ismoved vertically to engage the lower surface of the sheet material andmove the sheet material slightly upward to reduce, and preferably toeliminate, sag in the sheet material. For example, end effector 110 maybe moved upward about 5 mils after engaging the lower surface of thesheet material. Sag in the sheet material during testing is undesirableas such sag results in anomalous results in various tests. After thevertical movement, the vacuum hold downs 120 are engaged to maintaineach sensor 150 stationary relative to the surface of the sheetmaterial. The sensors 150 are then maintained in such proximity to testsites 155 to permit testing. The tests are then carried out. Withmultiplexing of the sensors, the tests are not necessarily carried outsimultaneously, but in series along each finger of end effector 110.Upon completion of the tests, the vacuum is released. End effector 110is moved vertically downward until it is no longer in contact with thesheet material. End effector 110 is then moved outward from the openingof cassette 130. End effector 110 is then positioned appropriatelyrelative to the next sheet to be tested in cassette 130, and the testsare conducted. The absence of a need for movement of the end effectorduring testing of a sheet improves throughput and reduces the risk ofdamage to the surface of the sheet material.

Referring to FIGS. 5, 6 and 7, there is shown an alternative embodimentof an apparatus of the invention for in-cassette testing of sheetmaterial. There is shown a cassette 230 according to the invention.Cassette 230 is essentially a rectangular box with an open front andwalls on its other five sides. The walls may be made from conventionallyused materials, for cassettes for use in storage of flat panels andwafers, such as metal or ceramics. Cassette 230 has a plurality of sheetshelves 240 for testing of sheet material. Each shelf 240 is a planarsheet of a rigid, non-contaminating material. For example, shelves 240may be made of one of various ceramics. Shelves 240 may be of metal,such as aluminum; suitable insulation may be provided between thealuminum of the shelf and the conductors. It may be desirable to useheavier conductors than laminated copper for better shielding; ifinsulated wires are used, the conductive nature of the aluminum is lessof a disadvantage. Each shelf 240 is rigidly supported on side walls ofcassette 230. Shelves 240 may be bonded to the side walls with anadhesive, be supported on projecting wires attached to the side walls,or otherwise be securely and rigidly supported. Each shelf 240 has aplurality of test heads or sensors 250 formed therein in an array. Thearray is preferably selected to be of a size relative to the surface ofmaterial to be tested to permit conducting of tests on test pointsincluding a substantial portion of the surface of the material. Sensors250 are preferably recessed at the level of the upper, planar surface ofshelf 240. Sensors 250 are arranged in several lines, although thepatterns of sensor locations may be varied. Vacuum hold downs 260 areprovided in pairs adjacent each sensor 250. Vacuum hold downs may beprovided in smaller or larger numbers or in different locations asdesired. Vacuum hold downs may be in the form of recesses or wells inthe body of shelf 240, which wells are in physical communication with atube. Each shelf 240 preferably has defined therein one or more cutoutsor recesses 270 intermediate rows of sensors 250. Recesses 270 aredefined to reduce the area of contact between shelf 240 and the sheetmaterial. Recesses 270 also reduce the weight of shelf 240, particularlyintermediate the walls of cassette 230. This reduction in weight tendsto reduce sag of shelf 240.

Exemplary wiring 280 is shown on the surface of shelf 240. Wiring 280may be placed on the lower surface of shelf 240, or interior to shelf240, as desired. In fact, each sensor 250 may have control and readoutlines associated therewith. Exemplary circuit boards 290, on whichappropriate control and memory electronics may be mounted, are shownimmediately below each shelf 240. The appropriate functionality toprovide control and memory for test data for the sensors may bephysically located elsewhere on the cassette, such as on an outersurface of a cassette wall. Alternatively, wiring 280 of shelves 240 maybe electrically connected during testing to electronics mountedexternally to cassette 230. Suitable connectors may be provided on eachshelf 240 for rapid connection and removal of connecting wiring. Byemploying externally mounted electronics, additional space is providedwithin cassette 230. Rather than providing a single board 290corresponding to each shelf 240, two or more shelves 240 in cassette 230may be multiplexed to a single board.

As many of the test devices or sensors provide analog data, the boardsmay include analog-to-digital converters to facilitate the exchange ofinformation with digital devices. Alternatively, the boards may havesolely analog electronics. The use of analog devices will reduce thecomplexity of the boards, although analog-to-digital converters will berequired remotely.

If photoreflectance testing is to be provided, in place of wiring to thesensors, optical fibers may be provided. A pair, one for emission ofradiation and one for detection of reflected radiation, may be providedat each test head. The optical fibers are preferably placed on the lowersurface of the shelf or in cavities defined interior to the shelf.Optical emitters and detectors may be provided on the shelf itself, onan associated board, or remotely.

Vacuum hold downs have tubes or pipes leading thereto from electricallyoperated valves, which in turn are connected to a pump, vacuum manifoldor the like. All hold downs on the surface of a shelf are preferably inphysical communication with a single valve, so that all vacuum holddowns on a surface engage and disengage the sheet materialsimultaneously. The valves may be mounted on the cassette, or may beexternal. Appropriate couplings are provided for tubes or pipes leadingfrom the vacuum hold downs. Couplings for vacuum lines may be providedto permit quick connection and release.

In an apparatus as shown in FIGS. 5-7, the sheet of material is placedin cassette 230 by a suitable robot end effector. The sheet is placed onthe surface of the ceramic shelf 240 to provide contact with the vacuumhold downs 260. If external electronics must be connected with thesheet, the connections are made. The vacuum hold downs 260, as a resultof a valve opening in response to a suitable signal from control boardor from external electronics, engage the sheet. The test is then carriedout. During the testing, the test heads 250 are caused to emit suitablesignals by control board 290 or external electronics, and to senseresulting fields, in accordance with well-known techniques. Uponcompletion of the testing, any electrical connections can be removed.The sheets remain in the cassette, and the cassette can be transportedto the location for the next processing step.

Referring now to FIGS. 8 and 9, there is shown an alternative embodimentof the shelf shown in FIGS. 5, 6 and 7. Shelf 340 is configured forconducting tests that require sensor or emitter devices on opposingsides of a panel or wafer. Shelf 340, as with shelf 240, includessensors or test heads 350, and vacuum hold downs 360 located in recessesin the upper surface of the shelf, and recesses 370. As shown in FIGS. 9and 10, the lower surface of shelf 340 includes test heads or sensors355 disposed in recesses formed in the lower surface. Test heads 355 arealigned with test heads 350. Suitably designed test heads 350 on a firstshelf and test heads 355 on a second shelf may be inductively coupledthrough a sheet or wafer to test various properties, in accordance withwell-known techniques. Test heads 350 and test heads 355 may be designedin other manners to cooperate to achieve testing of various properties.For example, test head 350 may emit radiation, and test head 355 maydetect radiation emitted by test head 350, or radiation emitted by thematerial as a result of exposure to radiation emitted by test head 350.Existing equipment designs need simply be miniaturized appropriately tofit in the recesses defined in the surface of shelf 340. The design ofFIGS. 8-10 may be readily incorporated in a cassette.

Also shown in FIGS. 9 and 10 are lines or tubes 375 running to vacuumdevices or hold downs. These lines 375 all physically communicate to asingle source. Lines 375 do not block sensors 355. As noted above, asingle valve may control communication with a vacuum manifold or pump.

Referring now to FIG. 11, there is shown in plan view an alternativeembodiment of a shelf according to the invention. In this alternativeembodiment, each test point 420 on shelf 440 includes two sensors 422,424 that are positioned in sufficiently close physical proximity to oneanother to cooperate in testing materials. For example, both testdevices 422, 424 may be coils that may be inductively coupled to eachother. Such inductive coupling permits measurement of variousproperties. Exemplary wiring 480 is shown. Each test point 420 alsoincludes pairs of vacuum hold downs or vacuum devices 450.

Referring now to FIG. 12, there is shown in a somewhat schematicisometric view a combined cassette and test location unit 500. Cassette505 is a conventional cassette, in the form of a rectangular box withside walls 510 having sheet supports 515 projecting horizontally inwardtherefrom. Front opening 520 is defined in cassette 505. Sheet supports515 are so positioned to permit a suitable end effector to fit betweenvertically adjacent sheets and between the sheet supports 515. Rigidlyattached to the lower wall of cassette 505 is testing location 525.Testing location 525 is also in the form of a rectangular box with afront opening. Testing location 525 includes test heads 530, which arerepresented schematically. Test heads 530 are preferably contactlesssensors for testing physical, mechanical, chemical or electricalproperties of sheets placed in test location 525. Such test heads areknown in the art. For example, test heads 530 may be sheet resistancesensors, which are positioned in vertically aligned pairs at the frontopening of test location 525. Suitable electrical connections tocontrollers and to data recording software and hardware are provided inaccordance with well-known techniques.

Also along the front opening of test location 525, intermediate testheads 530, there are provided vacuum chucks or vacuum hold downs 535.Vacuum chucks 535 are positioned to engage sheets and hold the sheetsmotionless relative to test heads 530 during each testing step. Vacuumchucks 535 also minimize sag of the sheets during testing. Suitablevacuum lines are provided in accordance with well-known techniques.Inward of front opening of test location 525 there are located substratesupports 540. The substrate supports 540 are elevated to support sheetswhen those sheets are placed in contact with vacuum chucks 535. Thesubstrate supports 540 are also sufficiently elevated above the floor oftest location 525 to provide clearance for a suitable end effector.

Representative end effector 545 is schematically shown adjacent testlocation 525. End effector 545 has two fingers 550. Fingers 550 are sopositioned and dimensioned, and test heads 530, vacuum chucks 535, andsubstrate supports 540 are all so positioned and dimensioned to permitfingers 550 to position a sheet on substrate supports 540 and vacuumchucks 535 in a position for testing by test heads 530.

In a method of testing sheet materials using unit 500, end effector 545,or another robot end effector, is employed to load sheets into cassette505. End effector 545 then is inserted into cassette 505, removes asheet, and moves the sheet into test location 525. When a desiredposition of the sheet in the horizontal plane relative to test heads 530is reached, end effector 545 moves downward to cause the sheet to besupported on vacuum chucks 535 and substrate supports 540. If desired,the sheet can be supported on end effector 545 as well. Vacuum chucks535 then engage the sheet, and testing is carried out by test heads 530.When testing is complete, vacuum chucks 535 disengage the sheet, and endeffector 545 engages the sheet again. If measurement at additionallocations is desired, end effector 545 moves in a suitable directionuntil the sheet is positioned properly relative to test heads 530, andthe above process is repeated. This process is repeated at as manylocations as desired. When the testing is complete, end effector 545removes the sheet from test location 525 and returns the sheet to astorage location in cassette 505. This process may be repeated for eachsheet in cassette 505.

The upper set of sensors may be mounted on a vertically movable mount.The mount may be guided on a vertical track. The mount in thisconfiguration is moved vertically by a stepper motor. This configurationis desirable if the amount of sag in the sheet before engagement by thevacuum chuck is greater than the desirable separation between upper andlower sensors. If this configuration is employed, the upper set ofsensors is in an upper position when insertion of the sheet by the endeffector commences. After insertion of the sheet and engagement by thevacuum chucks, the upper set of sensors is lowered to a lower position.In the lower position, the separation between upper and lower sensors isselected such that any effects of the separation are minimized.

In the embodiment of FIG. 12, it is also possible to mount sensors sothat they are movable in a horizontal direction. As it is notnecessarily possible to move the sheet laterally within the test area,lateral movement of sensors will permit additional portions of the sheetsurface to be tested.

It will be appreciated that the location of the sensors mounted relativeto the cassette reduces the distance that sheets must be moved. Also,the use of the end effector, rather than a separate handler, at the testlocation, reduces the number of pieces of equipment involved and thenumber of items of equipment that must contact the sheet.

In the embodiment of FIG. 12, rather than a robot end effector, ahandler positioned on an elevator may be employed to provide therequired vertical and horizontal movement of the sheet into and out ofthe cassette, between the cassette and the test area, and to variouspositions within the test area.

Referring now to FIG. 13, there is provided a top view of an alternativeshelf arrangement within a cassette. In this arrangement, there isprovided a partial shelf 605 at only the opening of the cassette. Thepositions of sensors 610 and vacuum hold downs 615 are representedschematically. Suitable electrical connections for sensors 610 and airlines for vacuum hold downs 615 may be provided. Exemplary sheetsupports 620 are also shown in the form of ledges protruding inward fromthe side and rear walls of the cassette. FIG. 14 is a partial frontcross-sectional view along line 14—14 of FIG. 13. FIG. 14 showscorresponding upper sensors 625. In this embodiment, an end effector,engaging a sheet, moves the sheet to a desired location relative tosensors 610 and 625. Vacuum hold downs 615 engage the sheet, andappropriate testing is conducted. Vacuum hold downs 615 then release thesheet, and the end 15 effector moves the sheet to a next desiredlocation. The process is repeated until a desired number of measurementshave been made. As the sensors and hold downs are located only at thefront opening of the cassette in this embodiment, repair and maintenanceare simplified. In this embodiment, single sensors may also be employedif desired.

In the embodiments with the sensors in the substrate and in the endeffector, either a coil cup, generally of metal foil, such as aluminum,may be used, or individual components may be separately fastened. A coilcup which supports all of the components is useful because it can beconstructed so that each sensor may readily be mounted and removed formaintenance.

In any of the embodiments, it will be understood that heads which usemicrowave radiation to excite the carriers of electric current, inducelocalized heating, and utilize a thermographic imaging system to measurethe physical, chemical, electrical and/or mechanical properties of thesample may be used.

It will be understood that testing is not confined to testing of wafersand panels merely after initial manufacture, but after any suitablestage in the process of manufacture of products incorporating wafers orpanels.

It will be appreciated that the foregoing method and apparatus may beused in connection with testing any material in sheet form, and is notlimited to use with the items, such as flat panels and semiconductorwafers, named as examples.

While an apparatus and method of the invention have been described withreference to particular embodiments, it will be understood thatadditional variations are within the scope of the invention.

What is claimed is:
 1. Apparatus for testing of material in sheet form,comprising: a cassette, comprising side walls and shelves rigidlysupported on said side walls, adapted to store one or more sheets ofmaterial; and one or more sensors for testing of properties of materialadjacent a test location exterior to said cassette rigidly mounted withrespect to said cassette, said test location comprising a defined spacefor receiving a sheet supported on an end effector.
 2. Apparatus ofclaim 1, wherein said cassette and said test location are so configuredand located to permit an end effector to move sheets of material betweensaid test location and storage locations in said cassette.
 3. Apparatusof claim 1, wherein said test location is below a lower wall of saidcassette.
 4. Apparatus of claim 1, wherein one or more vacuum hold downsare provided adjacent said test location.
 5. Apparatus of claim 1,wherein said sensors comprise compact test heads.
 6. Apparatus of claim1, wherein said sensors are adapted to emit microwave radiation. 7.Apparatus for testing of material in sheet form, comprising: a rigidsubstrate having a planar top surface and adapted to support thematerial, said substrate having a plurality of apertures definedtherein; a plurality of sensors located in said apertures below said topsurface and arranged in an array corresponding to a substantial portionof the surface of the material, said sensors being adapted to test atleast one of physical and chemical properties of the material at aplurality of points on the surface of the material.
 8. The apparatus ofclaim 7, wherein said substrate is of an electrically-insulatingmaterial.
 9. The apparatus of claim 7, wherein said sensors are adaptedto test sheet resistance of the material.
 10. The apparatus of claim 9,wherein said sensors are further adapted to employ eddy currents to testsheet resistance.
 11. The apparatus of claim 7, wherein emitters ofelectromagnetic radiation are located in said apertures.
 12. Theapparatus of claim 11, wherein receivers of electromagnetic radiationare located in said apertures.
 13. The apparatus of claim 7, wherein avacuum hold down is located in at least one of said apertures. 14.Method for testing properties of a sheet of material, comprising thesteps of: placing the sheet of material on an insulating, rigidsubstrate having a planar top surface; emitting electromagneticradiation toward the sheet simultaneously from a plurality of locationsbelow the top surface, the locations being arranged in an arraycorresponding to a substantial portion of the surface area of the sheetmaterial; detecting electromagnetic radiation received at the pluralityof locations.
 15. The method of claim 14, further comprising the step offixing the position of the sheet on the substrate before the step ofemitting.
 16. The method of claim 14, wherein sheet resistance of thematerial is determined based on the detected electromagnetic radiation.17. The method of claim 14, wherein the sheet of material is a siliconwafer.
 18. The method of claim 14, wherein the sheet of material is aflat panel for use in flat panel displays.
 19. The method of claim 14,wherein said step of placing comprises placing the sheet of materialstationary on the substrate, and wherein said step of emitting comprisesemitting electromagnetic radiation toward the sheet simultaneously froma plurality of emitters, one of said emitters being positioned at eachof said locations, and wherein said step of detecting comprisesproviding a detector at each of the plurality of locations.